1. Field of the Invention
The present invention relates to a device for detecting a visual line (line of sight) and to an optical apparatus capable of operating under control utilizing the detected visual line.
2. Description of the Related Art
In the following description, reference will be made to a camera which is one example of such an optical apparatus.
In recent years, with the rapid advance of electronic circuits or photoelectric conversion devices such as CCDs, the operations of cameras have increasingly been automated and various intelligent camera systems have been proposed.
It is said that this kind of automation has realized great improvements in that the operability of cameras has been greatly improved and in that even an ordinary person has become able to take a satisfactory photograph without the need of high photographic techniques.
However, the incorporation of automatic functions tends to impose limitations on the freedom of photography because of hardware limitations involved in the automatic functions. Accordingly, it is desired to provide a radical solution to this problem.
The most serious problem is that such an automatic camera is generally arranged in such a manner that its automatic control, whether automatic focus adjustment or automatic exposure control, can only function on a center-weighted basis with the result that a photographer is forced to select a photographic composition in which a main subject is located in the middle of a viewfinder screen.
Particularly in the case of focusing, it is necessary to accurately aim the camera at the main subject since it is photographically meaningless to focus all subjects on the viewfinder screen on the average. Accordingly, to succeed in photography, it is necessary to select a photographic composition in which the main subject is located in a position where an automatic focus detecting device can operate.
To solve the above-described problems, the concept of selecting a focus-determining area (distance-measuring point) on the basis of the visual line of a photographer is disclosed in Japanese Laid-Open Patent Application No. Sho 61-61135.
It has also been proposed to provide an apparatus which detects the position in an observing plane which an observer is observing, that is, an apparatus which detects a so-called visual line (visual axis).
For example, in Japanese Laid-Open Patent Application No. Sho 61-172552, it is stated that a parallel beam of light from a light source is projected onto the anterior segment of an eyeball of an observer and a visual axis is obtained by utilizing the position of a cornea-reflected image due to light reflected from the cornea and the image-forming position of the pupil. The visual-line detecting method will be described below with reference to FIGS. 19 and 20.
FIGS. 19 and 20 are views which serve to illustrate the principle of the visual-line detecting method. FIG. 19 is a schematic view of a visual-line detecting optical system, and FIG. 20 is an intensity diagram of an output signal from the photoelectric element array 6 shown in FIG. 19.
Referring to FIG. 19, a light source 5 is made from a light emitting diode for radiating infrared light to which an observer is insensitive, and is disposed in a focal plane of a light projecting lens 3.
Infrared light emitted from the light source 5 is collimated by the light projecting lens 3, and the obtained parallel beam of light is reflected by a half-mirror 2 and illuminates a cornea 21 of an eyeball 201.
At this time, a cornea-reflected image "d", which is formed by a portion of the infrared light reflected by the surface of the cornea 21, is transmitted through the half-mirror 2 and converged by a light receiving lens 4, whereby the cornea-reflected image "d" is formed at a position Zd' on the photoelectric element array 6.
Respective light rays from ends "a" and "b" of an iris 23 pass through the half-mirror 2 and the light receiving lens 4 and are focused at positions Za' and Zb' on the photoelectric element array 6 to form corresponding images of the ends "a" and "b".
If a rotating angle .theta. of an optical axis ii of the eyeball 201 with respect to the optical axis (optical axis i) of the light-receiving lens 4 is small, a Z coordinate Zc of a center "c" of the iris 23 is expressed as : EQU Zc=(za+Zb)/2
where Za and Zb represent the Z coordinates of the respective ends "a" and "b" of the iris 23.
If Zd represents the Z coordinate of a position where the cornea-reflected image "d" is formed and Oc represents the distance between a center of curvature, "O", of the cornea 21 and the center "c" of the iris 23, the rotating angle .theta. of the optical axis ii of the eyeball 201 substantially satisfies the following relation: EQU Oc.times.sin.theta.=Zc-Zd (1)
where the Z coordinate Zd of the position where the cornea-reflected image "d" is formed coincides with the Z coordinate of the center of curvature, "O", of the cornea 21. Accordingly, if the positions of individual singular points (the cornea-reflected image "d" and the images of the ends "a" and "b" of the iris 23), which are projected onto the photoelectric element array 6 as shown in FIG. 20, are detected, it is possible to find the rotating angle .theta. of the optical axis ii of the eyeball 201 by means of computing means 9.
In this case, the expression (1) can be rewritten as follows: EQU .beta..times.Oc.times.sin.theta.=(Za'+Zb')/2-Zd' (2)
where .beta. is an image-forming magnification and is normally an approximately constant value. The image-forming magnification .beta. is determined by a distance L1 between the position where the cornea-reflected image "d" is formed and the light receiving lens 4 and a distance L0 between the light receiving lens 4 and the photoelectric element array 6.
As is known, if the surfaces of optical systems such as the cornea of the eyeball and the crystalline lens are respectively regarded as spherical surfaces, the optical axis of the eyeball can be determined by connecting the centers of the respective spherical surfaces. However, when a person is actually observing an object, the person's eye is fixed on a point on an extended line of a line which connects the central fovea of the retina and the anterior nodal point. As a result, a certain extent of deviation occurs between the optical axis of the eyeball and the visual axis thereof, depending on a personal error peculiar to the observer.
As a proposal for solving this problem, Japanese Laid-Open Patent Application No. Hei 1-274736 discloses the art of correcting the angle deviation between the optical axis and the visual axis of the eyeball of the observer and detecting the visual line thereof.
If the horizontal rotating angle .theta. of the optical axis of the eyeball of the observer is calculated and angle correction .delta. is applied to the angle deviation between the eyeball optical axis and the visual axis, the horizontal rotating angle .theta.H of the visual line of the observer is expressed as: EQU .theta.H=.theta.+.delta.
where the sign .+-. is determined in such a way that if the sign + represents the angle of rightward rotation with respect to the observer, the sign + is selected when the observer's left eye is used to look into an observing device and, when the observer's right eye is used to look into it, the sign - is selected.
The vertical rotating angle of the visual line of the observer coincides with a vertical rotating angle .theta.' of the optical axis of the eyeball.
However, in the case of the above-described angle correction relative to a personal error associated with visual-line detection, a photographer must take the trouble to set a correcting mode for executing personal error correction. In addition, each time photographers change, it is necessary to set the correcting mode to again perform personal error correction.
When a plurality of photographers are operating a single camera by turns during photography, if a particular photographer forgets to perform a personal error correcting operation, operation control utilizing the visual line will cause an erroneous operation of the camera.